In a conventional optical recording medium, it is not possible to read a recording mark train whose period with a spatial period equal to or less than a certain length. The length of this recoding mark train period is referred to as a diffraction limit. In a reproduction optical system with a wavelength λ and a numerical aperture NA, the diffraction limit is λ/2NA.
The spot size of a full width half maximum (FWHM) is 0.59λ/NA at the diffraction limit.
If the length of the recording mark is equal to that of a blank area in one period, the length of recording mark is λ/4NA, which is referred to as a resolution limit. The reproduction of the information from such recording marks was impossible since a radiated beam could not distinguish recording marks smaller than λ/4NA.
In order for an optical medium to increase the recording density thereof, it is necessary to reduce the wavelength λ and/or to increase the numerical aperture NA. However, there are practical limits in changing λ and/or NA.
Therefore, a super-resolution optical medium containing a single layer of nonlinear material, has been proposed in U.S. Pat. No. 5,153,873, from which a recorded mark having a size of below the resolution limit can be reproduced.
The super-resolution techniques make it possible to reproduce recorded marks having a size of below the limit of resolution by way of using the optical property of the super-resolution material which changes depending on the intensity of the incident beam. Thus, a super-resolution storage medium can increase the recording density and capacity of an optical recording medium without shortening the wavelength λ of the incident beam and the numerical aperture NA of the objective lens.
For the super-resolution techniques to be applicable to any type of optical memories i.e. read-only type, write-once-read-many type and rewritable type, the super-resolution materials are desired to have optical transmittance increasing with an intensity of the incident radiation. Certain semiconductor materials, chalcogenide materials in particular, appear to be the most promising materials of these kinds. These materials exhibit super-resolution properties by way of absorption of an incident laser beam and the subsequent generation of heat leading to modulation in optical property.
Thus, regardless of the memory type, repetitive heating of the medium, involved in repetitive recording and reproducing of information, along with the consumption of laser power is inevitable. Accordingly, there is a need for improving durability of super-resolution media against repetitive heating while providing a higher carrier-to-noise (C/N) ratio at a lower laser power.